Continuous food sterilization system with hydrostatic sealed treatment chamber

ABSTRACT

Apparatus and method for continuously cooking and sterilizing particulate food material are disclosed wherein the outlet of a pressurized steam treatment chamber is sealed by hydrostatic sealing means in which sterilized, particulate food material is simultaneously cooled and depressurized as it is removed from the treatment chamber.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention pertains to apparatus and method useful in the asepticcanning of particulate food material. More particularly, this inventionrelates to apparatus and method for continuously cooking and sterilizingparticulate food material without substantially altering its texture,taste and nutritional value.

2. Description of the Prior Art

In the conventional process for canning foods, containers are firstfilled with food product and sealed. Thereafter, the sealed containersare heated in a pressure cooker or retort to sterilize the cannedproduct. Inadequate preservation of the canned food material'sorganoleptic quality is one well-recognized problem associated with thisapproach. To insure that every food piece is adequately sterilized,unavoidable overheating of at least some of the food pieces occurs. Thisis particularly true when some of the canned food pieces requiredifferent heating times to reach sterilization conditions. Inparticular, those foods requiring less time for sterilization generallytend to be overheated. Heat transfer limitations with the conventionalsterilization approach represents still another drawback. Sterilizingfood material by transferring heat through both a container and acontained fluid requires more energy than would otherwise be consumed ifthe canned material were sterilized directly.

Recognizing these limitations, the prior art has proposed procedures foraseptically canning food material. In aseptic canning, food material issterilized before it is sealed in a container. Generally, food materialis quickly heated to sterilization temperatures, typically in the rangeof 250° F. to 300° F., by direct contact with pressurized steam. Thefood material is maintained at such temperatures for sufficient time toeffect sterilization. Thereafter, the food material is rapidly cooledand the cooled, sterile material is filled into pre-sterilizedcontainers and sealed within a sterile or aseptic environment.

In a particularly efficient and convenient sterilization arrangement,food material is conveyed through a pressurized steam treatment chamber.In this arrangement, heat treatment is controlled simply by controllingthe rate food material is passed through the treatment chamber. In orderto insure the food material rapidly achieves sterilization temperatures,a temperature in the range of 250° F. to 300° F. must be maintained inthe treatment chamber. This condition is obtained using superatmospheric pressure steam, at pressures of, for example, about 10 toabout 55 psig.

To date, the basic arrangement for feeding solid particulate foodmaterial into and withdrawing it from a pressurized steam treatementchamber involves some type of rotary valve. Both inlet and outlet valvesact as seals to maintain pressure within the treatment chamber. Oneproblem with rotary valves, however, is that structural degredation ofthe product often occurs as a result of mechanical abrasion. This isparticularly true at the outlet valve where the texture of the foodproduct, to some extent, has been unavoidably impaired by sterilizationand is consequently more suseptible to mechanical damage. Anotherdrawback of rotary valves is that at the ever increasing processingspeeds demanded by food processors, there often is insufficientresidence time within the outlet valve for adequately cooling the foodmaterial to a temperature that avoids product flashing. Flashing iscaused by rapid vaporization of hot liquid from within the food materialcaused by its sudden depressurization. Flashing, which tends todisintegrate the solid food particles, can only be avoided by coolingthe food material below the atmospheric boiling point of absorbed liquidbefore depressurization. An improved apparatus and method for removingsolid food material from pressurized steam treatment chambers would bevery useful to the food processing industry.

It is an object of this invention to provide apparatus and method forremoving solid particulate food material from a pressurized steamtreatment chamber.

It is another object of this invention to provide apparatus and methodfor removing solid particulate food material from a pressurized steamtreatment chamber while avoiding product flashing.

It is still another object of this invention to provide apparatus andmethod for removing solid particulate food material from a pressurizedsteam treatment chamber while preventing its disintegration, attritionand mushing.

Other objects and advantages of the invention will become apparent fromthe following description.

SUMMARY OF THE INVENTION

In one aspect, the present invention relates to apparatus forcontinuously heat treating particulate food material with steam at superatmospheric pressure of the type having a steam treatment chamber withpressure-tight inlet and outlet openings through which particulate foodsare fed into and discharged from the treatment chamber.

The invention particularly relates to an improvement wherein ahydrostatic sealing means seals the outlet opening of the treatmentchamber. The hydrostatic sealing means comprises a column of liquid incommunication with the treatment chamber's outlet opening as well aswith a region of lower pressure, whereby the liquid column balances thesteam pressure in the treatment chamber. The liquid in the hydrostaticsealing means constitutes a broth for the treated particulate foodmaterial. The apparatus also includes means for cooling liquid in thehydrostatic sealing means and means for conveying particulate foodmaterial discharged from the treatment chamber through the hydrostaticsealing means to the region of lower pressure. By this arrangement, hotfood products discharged from the treatment chamber are cooled andgradually depressurized so as to avoid flashing and loss of structuralintegrity while extraction of valuable components such as vitamins,minerals and flavors from the food material is simultaneously prevented.

In another aspect, the present invention relates to a method for coolingparticulate food material discharged from a pressurized steam treatmentchamber having pressure-tight inlet and outlet openings comprising:

(a) providing hydrostatic sealing means to seal the outlet opening ofthe treatment chamber, said sealing means comprising a column of liquidin communication with the outlet opening and with a region of lowerpressure, the liquid in the liquid column balancing the steam pressurein the treatment chamber, the liquid constituting a broth for thetreated particulate food material;

(b) discharging particulate food material from said treatment chamberinto said liquid column;

(c) cooling the liquid in the hydrostatic sealing means; and

(d) conveying said discharged particulate food material through saidliquid column to the region of low pressure so that the temperature ofabsorbed liquid in said food material is always below its boiling pointat the pressure prevailing around the food material; whereby theparticulate food material is cooled and gradually depressurized as it isconveyed through the liquid column.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 schematically illustrates apparatus for practicing the presentinvention.

FIG. 2 illustrates a preferred arrangement for cooling liquid in thehydrostatic sealing means.

DETAILED DESCRIPTION

The phrase "particulate food material" is intended to embrace a widevariety and size of solid food materials from small vegetable piecescommonly used as garnish in soups, such as carrot, celery, onion andpotato dices and corn kernals, to whole food items such as broccoli andasparagus spears and cauliflower florets. Fruits, meats and seafoods arealso included within the intended meaning of food material. Generally,the size of the food material treated with this invention is determinedby final product considerations. This phrase is intended to excludeliquid and semi-solid food materials such as tomato sauce.

Referring to FIG. 1, particulate food material is delivered topressurized steam treatment chamber 10 through line 1. Particulate foodmaterial is temporarily stored in surge bin or feed hopper 2 and iscontrollably metered into chamber 10 through valve assembly 3.Generally, valve assembly 3 is a rotary type valve typically having acylindrical, closed-end shell which in combination with a driveninternal rotor defines moving pockets which serially advance meteredamounts of particulate food material from hopper 2 into treatmentchamber 10.

Pressurized steam treatment chamber 10 can be of any conventionaldesign. Generally, treatment chamber 10 will comprise a horizontal orslightly inclined cylindrical shell adapted to withstand an internalpressure between about 10 to about 55 psig. Chamber 10 will normally beoperated at a pressure of about 15 psig. High temperature, pressurizedsteam is fed into treatment chamber 10 through conduit 6 which typicallyincludes a pressure control valve for regulating the supply pressureindependently of a higher source pressure. Spent steam is withdrawnthrough pressure release valve-controlled conduit 7. Chamber 10 alsoincludes means for transporting particulate food material from inletopening 4 to outlet opening 5. This can be done simply by inclining thechamber 10 and rotating or oscillating it on its axis to thereby causeparticulate food material to tumble through the chamber. Preferably,chamber 10 is provided with means for positively urging food materialtherethrough, such as a helical conveying screw or conveying paddles.These latter arrangements allow better control of the food material'sresidence time in the treatment chamber and also tend to cause lessabrasion. Adequate control of residence time is particularly importantif continuous sterilization is to be obtained without seriouslyimpairing the food materials organoleptic quality. The food material isnormally subject to a steam temperature above about 121° C. for at leastabout 20 minutes. In the broad practice of this invention, however,steam temperatures from above about 100° C. are contemplated.

Heat treated, particulate food material is discharged from treatmentchamber 10 into the liquid column 8 of hydrostatic sealing means 20.Obviously, the steam treatment temperature will determine thetemperature of the discharged food material. Typically, the temperatureof the food material will be about 120° C. In this preferred embodiment,liquid column 8 of hydrostatic sealing means 20 comprises an innerportion 11 interconnected with an outer portion 12. The inner portioncommunicates with outlet opening 5 of treatment chamber 10, while theouter portion communicates with low pressure region 15. The height ofthe liquid column 8 is sufficient to balance the steam pressure intreatment chamber 10. In order to seal a treatment chamber operating atpressures between about 10 to about 55 psig., liquid column heightsbetween about 20 and about 130 feet are required. The liquid heightneeded to seal the treatment chamber at any particular operatingpressure is readily apparent to one skilled in the art.

Particulate food material descends through inner portion 11 and iscollected by endless conveyer 13. Conveyer 13 can be of a conventionaldesign and transports the particulate food material upwardly throughouter portion 12. Other arrangements for conveying the food materialthrough the hydrostatic sealing means will be apparent to those skilledin the art. For example, a screw conveyor could be substituted forendless conveyor 13. The food material is cooled and graduallydepressurized as it is conveyed upwardly through outer portion 12.

Generally, the treated food material will be conveyed through thehydrostatic sealing means to an atmospheric pressure region.Consequently, to avoid product flashing the temperature of absorbedliquid in the treated food material must be less than 100° C. as itexits the hydrostatic sealing means. More precisely, to prevent productflashing at every point in the food material's travel through thehydrostatic sealing means, the temperature of absorbed liquid in thetreated food material must be below its boiling point at the pressureprevailing around the food material. Liquid in the sealing means must bemaintained at an appropriate temperature to provide this result. Sincehot food material is continuously discharged into and conveyed throughthe hydrostatic sealing means, the liquid in hydrostatic sealing means20 must be cooled. While a wide temperature distribution is possibledepending upon such factors as the nature of the food material beingtreated, the food material's exit temperature from treatment chamber 10,the speed of conveyor 13, etc., the liquid in at least the upper sectionof outer portion 12 is generally maintained at about 95°-98° C. whilethe liquid at the bottom of outer portion 12 is generally maintained atabout 115°-125° C. At these conditions, the speed of conveyor 13 isgenerally regulated so that it takes a few minutes, e.g., about 2-6minutes, for the food material to be conveyed through the hydrostaticsealing means.

An appropriate temperature distribution for cooling food material can bemaintained in hydrostatic sealing means 20, for example, by deliveringcooled liquid thereto through conduit 16 and withdrawing hot liquidtherefrom through conduit 17. Alternatively, liquid in hydrostaticsealing means 20 can be cooled by locating appropriately designedindirect heat transfer surfaces; e.g., cooling coils, within liquidcolumn 8, preferably in outer portion 12. In any event, the speed ofconveyor 13 is controlled such that as particulate food material isgradually depressurized, and simultaneously cooled, the temperature ofabsorbed liquid is always below its boiling point at the pressureprevailing around the food material. In this way, product flashing isprevented. The combination of high-efficiency direct liquid cooling andthe extended path traveled by conveyor 13 through hydrostatic sealingmeans 20 permits high-speed continuous operation without creating aproduct flashing problem.

An important aspect of this invention is its use of broth as the liquidin hydrostatic sealing means 20. As used herein, the term "broth"broadly refers to an aqueous liquid having a concentration of vitamins,minerals, flavor constituents, etc., that retards or prevents theextraction of water soluble components from a particulate food materialas it passes through the hydrostatic sealing means. By employing brothas the cooling liquid, the taste and nutritional value of food materialcan be better preserved. The necessary broth can be conviently developedby recirculating liquid in the hydrostatic sealing means through aclosed loop, as will be more fully described hereafter in connectionwith FIG. 2.

Endless conveyor 13 is designed so that liquid can drain therethrough asit emerges from the outer liquid column 12. In this way, free liquid ispermitted to drain from the particulate food material before it isdischarged from conveyor 13 into duct 21.

Cool and depressurized, treated food material is discharged from theupper end of conveyor 13 falling through duct 21 to sterile or asepticfilling zone 30. Duct 21 can be provided as a column and relatively dry,sterile gas can be flowed upwardly therethrough if a dry product isdesired for canning. Sterilized containers are fed to zone 30, asindicated at 31, and filled containers are discharged therefrom asindicated at 32. If desired the filled cans can then be sealed, eitherbefore or after filling with additional ingredients.

A preferred way of cooling liquid in hydrostatic sealing means 20 isillustrated in FIG. 2. Hot liquid withdrawn through conduit 17 fromliquid column 8 at inner portion 11 is passed through conduit 117 topump 118. The liquid is pumped through filter 119 to remove large solidsand makeup liquid is then added through valved-conduit 120 as requiredto maintain the hydrostatic seal. The liquid is then sterilized byindirect heat exchange with steam in heater 121. The liquid is held atthe required sterilization temperature in holding tube 122 and isthereafter cooled by indirect heat exchange with cooling fluid incooling zone 123. The degree of cooling is controlled by temperaturerecorder-controller 140 which senses the temperature of the cooledliquid through sensing means 141 and appropriately adjusts the coolingfluid flow rate via line 142 and valve 143. A quantity of liquid isdiverted as required from the cooling circuit through valved-conduit 124to prevent the excessive buildup of certain constituents in the coolingcircuit. Additionally, if desired, broth can be continuously divertedthrough conduit 124 for use as a liquid filler or sauce in canning theparticulate food material. Cooled liquid is then returned to thehydrostatic sealing means through conduit 116 and is injected into thesealing means through conduit 16. By this arrangement, valuableconstituents, for example flavors, vitamins and minerals, extracted froma particulate food material are equilibrated in the liquid. Thiscondition prevents further leaching of such components from theparticulate food material. As another important feature, the circulatingliquid can be monitored for salt level and pH and appropriately adjustedas desired.

While preferred embodiments of this invention have been discussedherein, those skilled in the art will appreciate that changes andmodifications may be made without departing from the spirit and thescope of this invention, as defined in and limited only by the scope ofthe appended claims. For example, while the present invention has beenspecifically described with respect to the sterilization of particulatefood materials, and has been shown to have particular utility therein,it is anticipated that the invention may also have applicability to thecontinuous heat treatment of food materials solely to inactivate enzymesand destroy molds which is done at somewhat lower temperatures andpressures.

I claim:
 1. A method for cooling particulate food material dischargedfrom a pressurized steam treatment chamber having pressure-tight inletand outlet openings comprising:(a) providing hydrostatic sealing meansto seal the outlet opening of the treatment chamber, said sealing meanscomprising a column of liquid in communication with the outlet openingand with a region of lower pressure, the liquid column having a heightin the range of about 20 to about 130 feet for balancing a steampressure in the range of about 10 to about 55 psig in the treatmentchamber, said liquid constituting a broth for the treated particulatefood material; (b) discharging particulate food material from saidtreatment chamber to said liquid column; (c) cooling the liquid of thehydrostatic sealing means; and (d) conveying said discharged foodmaterial through said liquid column to said region of lower pressure sothat the temperature of absorbed liquid in said food material is alwaysbelow its boiling point at the pressure prevailing around the foodmaterial whereby the particulate food material is cooled and graduallydepressurized as it is conveyed through the liquid column.
 2. The methodof claim 1 wherein the particulate food material is selected from thegroup consisting of meats, vegetables, fruits and seafoods.
 3. Themethod of claim 1 wherein the particulate food material is dried bydirect contact with a relatively dry gas in said region of low pressure.4. The method of claim 3 wherein the dried, particulate food material ispassed to an aseptic filling zone.